The invention relates generally to monitoring fluid in a line without direct fluid contact, and more particularly, relates to non-intrusively monitoring for the presence of air in that fluid.
In numerous medical and industrial applications, continuous in-line monitoring of a fluid is often necessary to ensure consistency of a process or to ensure safety. For example, the presence of air within a fluid may need to be monitored for various reasons. Examples of non-medical applications for fluid monitoring without direct fluid contact can be found in the chemical process industry, where inexpensive and/or disposable fluid conduits may be required, where fluids may be present at high pressure, or where fluids that are highly caustic or highly toxic may be involved.
In the medical area, safety and cost are of great concern. Reliable and inexpensive in-line fluid monitoring without direct fluid contact is especially important in the medical area where sterilization is required and disposability of the fluid line is desired. One particularly important purpose of in-line fluid monitoring is the detection of air. Air-in-line detection systems are used to prevent the inadvertent infusion of a dangerous quantity of air into a patient's bloodstream. While small bubbles of air may have no adverse effect on a patient, large air bubbles can cause death. The amount of air that is dangerous can vary and depends on patient characteristics; therefore, the ability to detect various sizes of air bubbles is desirable.
Methods and systems for the in-line detection of air have typically involved ultrasound or light transmission through the fluid line being monitored. The transmission characteristics of sound or light may be utilized as an indication of the presence of a gas bubble in liquid in the fluid line. Simple recognizable perturbations of the signals from such sensors may be utilized to trigger an alarm and/or halt the infusion. Such systems require that the fluid and the associated conduit be substantially transparent to the energy being transmitted. However, in some cases the detection system is unable to reliably distinguish between air bubbles of varying sizes, resulting in erratic behavior with false indications of the presence of air bubbles. Typically, such detection systems do not accurately determine the exact size of air bubbles and are configured merely to indicate the presence of air bubbles that are greater than a predetermined size.
In optical systems, extraneous light reaching the optical detector can compromise the system's accuracy. In acoustic frequency systems, good mechanical coupling of the transducers to the fluid line is of large importance. Poor mechanical coupling will mask from the detection system the case where the air detected is located between a transducer and the fluid line rather than actually within the fluid line. Consequently, the false alarm rate may be unacceptably high. As a result of this consideration, much effort and expense have gone into the mechanical design of the detection system coupler to obtain good mechanical coupling. Such systems may occasionally still have less than desirable mechanical coupling where the fluid line sizes vary or foreign materials become stuck to the outside of the fluid conduit and come into contact with the coupler.
A further consideration in air-in-line detections systems is the determination of the exact volume of the air bubble in the fluid line in which fluid is moving. Many systems use transducers that are smaller than the volume of air in the fluid conduit that would pose a danger to the patient. Some such systems "time the bubble." That is, once a quantity of air is detected, a timer is initiated to determine the amount of time that the detector "sees" the air. A processor then calculates the bubble size based on the internal volume referenced to the transducer size, the flow rate, and the time the bubble is detected. Processing is made more complex in the case where a train of bubbles exists where the bubbles are interspaced with liquid and the transducers alternately indicate air and liquid. Such processing can result in a large increase in expense and complexity.
Other apparatus capable of detecting impurities such as air within a fluid include electrochemical systems and laser doppler systems. Electrochemical systems can be extremely sensitive to specific compositional variations in a fluid, but incorporate components, such as membranes, that must be in direct contact with the fluid, thus increasing their costs in applications requiring disposability. Laser systems are at present very expensive, and still other systems cannot operate over the wide range of flow rates and fluid types required in many applications.
Hence those concerned with fluid line monitoring have recognized that it would be beneficial to provide an in-line fluid monitoring system and method that do not involve direct fluid contact with a sensor but that exhibit higher sensitivity to variations in fluid composition, including the presence of air and that can provide an indication of the size of an air bubble. In medical systems, there is a need for an apparatus and a method that reliably and accurately detect and quantify the presence of air in the line but at the same time are relatively inexpensive and can function with an inexpensive disposable fluid line. The present invention fulfills these needs and others.